Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 292101
AFDBIS

AREA FORECAST DISCUSSION
National Weather Service Bismarck ND
401 PM CDT SUN MAY 29 2016

.SHORT TERM...(This evening through Monday)
Issued at 401 PM CDT Sun May 29 2016

Thunderstorm potential on Memorial Day Monday and Monday evening
is the focus of the short term forecast.

First of all, weak convection is occurring across southwest ND
this afternoon in association with a weak shortwave trough. Recent
convection-allowing model guidance continues to support the slow
east-northeast propagation of these weak cells into the evening,
when boundary layer cooling causes increasing CIN for surface-
based parcels near the time that the shortwave exits the area. We
then carried low-end PoPs for showers and thunderstorms overnight
in parts of western and central ND in respect to slowly increasing
MUCAPE values and weak, transient signals in various convection-
allowing model simulations for such activity. However, the low-
level jet is forecast to be weak, with 850-MB winds peaking near
25 kt, and broad mid-level height rises also suggest the odds of
convection occurring before daybreak may be minimal.

On Memorial Day Monday, a strong shortwave trough will approach
the state, causing a surface low to develop over western SD. The
concomitant increase in low-level warm and moisture advection is
expected to yield increasing instability, especially in southwest
and south central ND where surface dewpoints are forecast to rise
into the 55-60 F range per a consensus of 12 UTC guidance. Height
falls will commence over western ND by midday and by that time it
is possible the boundary layer will be uncapped, perhaps allowing
for convection to deepen and expand rather early in the day before
return flow has had adequate time to sufficiently moisten and thus
destabilize the air mass. Moreover, anvil blowoff from early-day
convection could stunt downstream warming, also reducing CAPE and
perhaps marginalizing subsequent severe potential, or at the very
least reducing its real extent. Forecast soundings taken in the
pre-convective environment do favor around 1500 J/kg of MLCAPE,
mid-level lapse rates exceeding 7 C/KM, and 0-6-km bulk wind shear
of 30 t0 40 kt, even in the models that do develop convection by
midday. That is sufficient for organized/severe storms posing a
large hail and damaging wind threat where bouyancy is greatest.
Guidance does suggest potential for greater MLCAPE exceeding 2000
J/kg in far south central ND if the boundary layer is allowed to
warm and moisten long enough into the afternoon, again favoring a
greater severe weather threat if storms hold off long enough.

The 12 UTC GFS and NAM, and to a lesser extent 12 UTC ECMWF, favor
an early evolution to storms. However, convection-allowing model
guidance shows disparity in possible timing, as the NCEP-run WRF-
ARW develops storms the quickest and takes them the furthest east
by 21 UTC, while the NCEP WRF-NMM and 4 km NAM Nest are both
slower with deep convective onset, favoring a greater risk of
severe weather, especially in south central ND. The NSSL WRF-ARW
is a medium-ground solution. All of this goes to show that there
is rather considerable uncertainty in the details of the severe
potential. Thus, at this juncture, the only area where we included
an explicit mention of severe thunderstorms within the gridded
and point-and-click forecast is Grant, Sioux, southern Morton,
and Emmons Counties. It`s in that area where calibrated severe
thunderstorm probabilities from the 09 UTC SREF are maximized at
noteworthy values.

.LONG TERM...(Monday night through Sunday)
Issued at 401 PM CDT Sun May 29 2016

Cool and potentially wet weather on Tuesday will transition to
warm and mainly dry weather by late week and next weekend.

A 500-MB low is forecast to cross ND on Tuesday, and a shield of
showers and weak thunderstorms will follow the low, with a focus
to its west and north where large-scale forcing will be greatest.
The 00 and 12 UTC NAM solutions are further north than the ECMWF
and GFS simulations with that low. Given the clustering of those
latter two global models, which typically verify better with the
500-MB height fields (even with dynamic waves) at 48+ hours, the
official forecast was weighted toward those ECMWF and GFS ideas.
That means we are carrying 60+ percent PoPs Tuesday over western
and north central ND,and a bit lower PoPs over south central ND in
closer proximity to a potential dry slot. Clouds and showers will
generally keep instability at bay and so severe storms are not
expected. However, in closer proximity to the dry slot tied to the
mid-level low, the 12 UTC guidance suggests 0-3-km SBCAPE may
exceed 150 J/kg, suggesting a non-zero risk of funnels/weak cold
core tornadoes in parts of central ND given ambient/environmental
vorticity. Having said that, those situations are notoriously
difficult to predict, especially beyond 12 hours, and taken at
face value neither the GFS or NAM suggest VGP values great enough
to suggest this threat is anywhere near great enough for any
mention in public hazardous weather outlooks at this time.

Thereafter, we have strong confidence in a reshuffling of the flow
aloft with the main feature of interest a stout ridge aloft that`s
expected to become established over the Great Basin late this week
and next weekend. This favors a warming and drying trend over the
area, with highs expected to reach the 80s Friday through Sunday
based on the 12 UTC multi-model consensus. Northwest flow aloft on
the downstream side of the ridge suggests a mainly dry regime, and
around 50% of the CIPS analog cases derived using the GFS ensemble
support below-normal precipitation in that time period. That said,
the past few GFS and ECMWF runs have shown modest agreement with a
weak northwest-flow shortwave trough passage around Friday, which
could yield a low chance of showers and thunderstorms near then.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1250 PM CDT Sun May 29 2016

VFR conditions are expected today with mainly a VFR cumulus field
developing. There is a small chance of afternoon/evening showers
or thunderstorms in the far southwest, but chances too small to
even mention in KDIK taf yet. Late tonight and after 12z
Monday...scattered showers and thunderstorms are expected to
develop...becoming widespread around/after 18z.

&&

.BIS Watches/Warnings/Advisories...
NONE.
&&

$$

SHORT TERM...CJS
LONG TERM...CJS
AVIATION...JV


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